MOO 


WALLri         I    LLA   A  .  .II 

By    James  H.    V.'eir 

te  print    from   Journal   of  Agricultural 
.iesearch,    Vol.    IV,    No.    4,    July  15,    1915 


MffiOFAGRIClMAL 

«7 


VOL.  IV  WASHINGTON,  D.  C.,  JULY  15,  1915  No.  4 


ARCKUTHOBII 
By  JAMES  R. 


' 


AGRIC. 
LIBRARY 

' 

WALIvROTHIELLA  ARCEUTHOBII 


By  JAMES  R.  WEIR,  ™    , 

Forest  Pathologist,  Investigations  in  Forest  Pathology,  ,<•*'• 

Bureau  of  Plant  Industry 

INTRODUCTION 

The  fungus  Wallrothiella  arceuthobii  1  has  the  distinction  of  causing  the 
only  disease  of  the  leafless  mistletoes  so  far  described.  Comparatively  few 
botanists  have  seen  this  interesting  fungus  either  in  nature  or  in  museums, 
and  all  reference  to  it  has  been  omitted  from  works  dealing  strictly  with 
plant  diseases.  The  fungus  was  apparently  never  collected  again  from 
the  region  where  it  was  originally  discovered,  and  no  record  exists  of 
preserved  material  from  the  place  of  its  second  discovery.  The  dis- 
covery of  the  fungus  by  the  writer  in  the  Northwest  adds  a  new  interest 
to  its  study,  especially  since  it  is  found  to  be  so  abundant  as  to  have 
some  economic  significance. 

Owing  to  the  fact  that  the  fungus  has  only  been  reported  twice  and 
from  widely  separated  stations,  its  literature  is  very  meager.  It  was 
originally  discovered  by  Peck,2  who  published  a  short  account  of  it  under 
the  following  name  and  description : 

Sphaeria  arceuthobii,  n.  sp. 

Perithecia  small,  densely  caespitose,  oblong  or  cylindrical,  very  obtuse,  shining 
black;  asci  subclavate,  fugacious;  spores  crowded,  globose,  colorless,  .oooi6//  in 
diameter. 

Capsules  of  Arceuthobium  pusillum.     Forestburgh.     Sept.  (Plate  I,  figs.  10-14). 

It  forms  little  black  tufts,  crowning  the  fruit  at  the  tips  of  the  stems  and  branches. 
I  have  not  seen  it  on  the  staminate  plant.  I  am  not  fully  satisfied  that  the  generic 
reference  is  correct,  as  the  perithecia  seem  to  be  mouthless.  It  is  interesting  to 
observe  the  extent  to  which  parasitism  prevails.  The  Arceuthobium  is  a  parasite 
on  the  spruce,  this  fungus  is  parasitic  on  the  Arceuthobium,  and  in  a  few  instances  a 
third  parasite,  a  minute  white  mold,  was  seen  on  the  perithecia  of  the  fungus. 

The  second  discovery  of  the  fungus  was  by  Wheeler 3  in  the  Upper 
Peninsula  of  Michigan,  who  reports  it  as  follows: 

I  found  that  the  mistletoe  was  also  attacked  by  a  fungous  parasite,  which  must 
have  a  tendency  to  check  the  spread  of  this  pest.  Each  fruit  is  attacked  at  its  apex 
by  the  fungus  Wallrothiella  arceuthobii,  Peck,  and,  of  course,  destroyed. 

Both  Peck  and  Wheeler  published  some  excellent  drawings  of  the 
fungus,  from  which  a  very  good  idea  of  the  character  of  the  disease  may 
be  obtained. 

1  If  the  system  of  classification  of  Engler  and  Prantl  is  employed,  the  fungus  would  be  referred  to  Rosel- 
linia.     (Engler,  Adolf,  and  Prantl,  K.  A.  E.     Die  nattirlichen   Pflanzenfamilien.     T.  i,  Abt.    i,  p.  394, 
400,  404,  fig.  258,  A,  B.     Leipzig,  1897.) 

2  Peck,  C.  H.     Report  of  the  botanist.     In  27th  Ann.  Rpt.  N.  Y.  State  Mus.  Nat.  Hist.  1873,  p.  in. 
pi.  i,  fig.  10-14.     1875. 

3  Wheeler,  C.  F.    The  geology  and  botany  of  the  Upper  Peninsula  experiment  farm.     In  Mich.  Agr. 
Exp.  Sta.  Bui.  186,  p.  27-28,  4  pi.     1900. 

Journal  of  Agricultural  Research,  Vol.  IV,  No.  4 

Dept.  of  Agriculture,  Washington,  D.  C.  July    15,    1915 

G— 51 
(369) 


i 


370  Journal  of  Agricultural  Research  vol.  IV.NO.  4 

HOSTS 

The  interest  arising  from  the  remarkable  habit  of  this  fungus  of  attack- 
ing and  destroying  the  immature  fruits  of  Razoumofskya  pusilla  (Peck) 
Kuntze  (Arceuthobium  pusillum  Peck)  on  the  eastern  black  spruce  (Picea 
mariana)  has  led  the  writer  to  search  most  diligently  for  it  on  western 
species  of  Razoumofskya.  The  fungus  was  not  found  in  any  region  of 
the  Great  Lakes  States,  although  the  mistletoe  of  the  spruce  was  abun- 
dant and  much  material  was  examined.  In  the  West  the  search  has  been 
more  successful.  The  fungus  was  first  collected  on  Razoumofskya  doug- 
lasii  Englm.  (PI.  LV,  fig.  i)  in  the  vicinity  of  Como  Lake  in  the  Bitter- 
root  Mountains,  Montana.  The  only  tree  found  bearing  infected  plants 
stood  in  .a  clump  of  "left-overs"  on  a  cutting  area  of  the  Latchem  Lumber 
Co.  The  mistletoe  is  so  very  abundant  in  this  region  and  suppresses 
the  Douglas  fir  (Pseudotsuga  taxifolia)  to  such  an  extent  that  this  tree, 
according  to  Supervisor  White,  of  the  Bitterroot  National  Forest,  is 
sometimes  omitted  altogether  from  the  estimate  of  the  prospective  cut. 
Practically  the  same  conditions  prevail  throughout  the  entire  Bitterroot 
and  Missoula  River  Valleys  and  adjacent  regions.  It  has  always  been 
a  rule  of  the  writer,  in  regions  of  a  heavy  infection  by  R.  douglasii,  to 
look  for  the  small,  closely  related  mistletoe  designated  by  Engelmann 
"Razoumofskya  douglasii,  var.  abietina,"  on  Abies  grandis  and  A.  lasio- 
carpa.  Experience  has  shown  that  this  mistletoe  is  more  likely  to  occur 
when  its  hosts  are  in  the  vicinity  of  the  Douglas-fir  mistletoe.  Whatever 
conclusions  may  be  drawn  from  this  as  to  the  probable  relationship  of 
the  mistletoe  on  Abies  spp.  with  the  one  on  Pseudotsuga  spp.,  the  surmises 
as  to  the  presence  of  the  former  were  correct  in  the  present  instance. 

The  mistletoe  was  discovered  once  on  A .  grandis  by  the  large  spreading 
or  upright  brooms.  The  tree,  standing  not  more  than  100  feet  from  a 
Douglas  fir  which  supported  three  immense  brooms  of  R.  douglasii,1 
was  felled  and  the  mistletoe  plants  carefully  examined.  A  few  of  the 
plants,  which  were  pistillate,  were  found  to  be  infected  by  W.  arceuthobii 
(PI.  LV,  fig.  2).  On  another  portion  of  the  same  area  the  fungus  was 
discovered  on  a  few  plants  (PI.  LV,  fig.  3)  growing  deep  within  a  large 
broom  on  A.  lasiocarpa.  Two  other  mistletoe-infected  trees  of  this  spe- 
cies were  cut,  all  that  were  found  in  the  region,  but  the  parasite  was  not 
attacked  by  the  fungus.  A  search  in  other  sections  of  the  Bitterroot 
Valley  resulted  in  finding  the  mistletoe  on  the  grand  and  alpine  firs,  but 
the  fungus  was  not  present.  At  the  head  of  Rattlesnake  Creek,  a  small 
stream  flowing  into  the  Missoula  River  at  Missoula,  the  fungus  was  dis- 
covered on  a  small  mistletoe  (PI.  LV,  fig.  4)  growing  on  Picea  engelmanni. 
This  is  the  first  report  of  a  mistletoe  occurring  on  a  spruce  in  the  North- 

1  The  brooms  caused  by  this  mistletoe  are  sometimes  very  large  and  frequently  cause  the  death  of  the 
entire  crown  above.  The  infections  of  no  other  mistletoe  initiate  greater  and  more  frequent  brooming  than 
do  those  of  R.  douglasii.  For  this  reason  it  is  one  of  the  most  serious  parasites  of  the  entire  genus. 


July  15, 1915  Wallrothiella  A  rceuthobii  371 

west.  It  is  apparently  the  same  as  that  described  by  Engelmann  on 
spruce  from  the  Sierra  Blanca  Mountains  in  northern  Arizora  under  the 
name  "  R.  douglasii,  var.  microcarpa."  A  single  collection  of  what  is 
apparently  the  same  mistletoe  on  spruce  was  made  from  a  recently  fallen 
tree  on  a  cutting  area  near  Laclede,  Idaho.  The  parasite  was,  however, 
in  a  healthy  condition.  The  Douglas-fir  mistletoe  was  found  in  each  of 
the  two  last-named  regions.  The  fungus  material  in  all  the  foregoing 
cases  was' very  scanty.  It  is  believed,  however,  that  a  more  protracted 
search  will  result  in  finding  the  fungus  more  abundantly  on  the  Douglas- 
fir  mistletoe. 

In  order  to  test  the  ability  of  the  fungus  to  attack  other  mistletoes, 
infected  plants  of  R.  douglasii  were  bound  in  contact  with  pistillate 
plants  of  R.  americana,  the  mistletoe  of  the  lodgepole  pine  (Pinus 
murrayana).  To  prevent  accident,  the  experiments  were  protected  by 
binding  cheesecloth  loosely  about  the  stem  supporting  the  mistletoe, 
completely  inclosing  the  plants  but  not  interfering  with  their  vital 
functions.  In  all,  four  such  experiments  were  made  during  the  month 
of  October,  1913.  Since  pollination  had  already  taken  place  in  the 
early  spring,  it  was  inferred  that  the  fruits  of  the  lodgepole-pine 
mistletoe  would  mature  normally  if  infection  did  not  occur.  In  order  to 
have  fully  mature  plants  on  the  same  stem  for  purposes  of  comparison, 
small  tufts  just  below  the  inclosed  ones  were  shielded  by  a  circular  piece 
of  thick  white  cloth  tied  just  above  the  tuft  and  hanging  down  in  the 
form  of  a  loose  umbrella.  This  would  not  prohibit  the  circulating  spores 
from  coming  up  under  the  shield,  provided  they  escaped  from  the  cheese- 
cloth net,  but  would  lessen  the  chances  of  inoculation.  Furthermore, 
owing  to  ascending  air  currents,  spores  of  fungi  usually  travel  upward 
or  at  least  not  directly  downward  when  starting  from  an  elevated  point. 
Other  experiments  were  initiated  on  the  lodgepole-pine  mistletoe  by 
crushing  in  water  a  number  of  mature  perithecia  and  thoroughly  spray- 
ing the  mixture  containing  spores  over  a  few  pistillate  plants.  To  pre- 
vent the  plants  being  knocked  off  during  the  winter,  they  were  also  pro- 
tected by  cheesecloth.  These  experiments  were  visited  in  the  latter  part 
of  November,  1914.  The  results  were  positive.  As  shown  in  Plate  LV, 
figure  5,  not  only  had  the  fruits  of  the  lodgepole-pine  mistletoe  which 
were  inclosed  with  the  infected  plants  from  the  Douglas  fir  become 
infected  but  very  thoroughly  so.  Every  fruit  bore  at  its  apex  the  little 
shiny  black  tufts  of  the  perithecia  of  the  fungus.  One  fruit  shown  to 
the  right  in  the  middle  figure  of  the  sprayed  plants  seemed  to  have  escaped 
early  infection  and  to  have  attained  nearly  a  normal  size,  but,  neverthe- 
less, succumbed  to  the  parasite.  The  tufts  of  mistletoe  just  below  the 
infected  ones,  which  were  shielded  from  above,  did  not  become  infected 
and  produced  normal  mature  seeds,  which  were  being  expelled  at  the  time 
the  experiments  were  discontinued.  The  perithecia  of  the  fungus  on 


372  Journal  of  Agricultural  Research  vol.  iv,  No4. 

R.  americana  contained  mature  spores;  hence,  the  life  cycle  of  the  fungus 
in  the  seed  capsule  is  complete  in  the  fall  of  the  second  year  coincident 
with  the  time  required  for  the  ripening  of  the  seeds  of  the  host. 

Soon  after  the  conclusion  of  the  experiments,  a  swamp  area  in  the 
Kaniksu  National  Forest,  Idaho,  scatteringly  timbered  by  lodgepole 
pine,  was  visited.  The  trees  were  heavily  infected  with  the  mistletoes 
characteristic  of  this  tree.  A  close  examination  of  the  mistletoe  plants 
showed  them  to  be  uniformly  attacked  by  the  fungus  throughout  the 
entire  area.  So  abundant  was  the  fungus  that  very  few  of  the  pistillate 
plants  on  any  of  the  trees  had  escaped  attack.  This  area  has  been  a 
fruitful  source  of  investigation  and  a  number  of  important  facts  have  been 
gathered. 

SIGNIFICANCE  OF  THE  FUNGUS  TO  THE  TAXONOMY  OF    ITS    HOSTS 

The  hosts  of  W.  arceuthobii,  so  far  as  known  at  present,  are  as  follows: 
Razoumofskya  pusilla  (Peck)  Kuntze  on  Picea  mariana. 
R.  americana  (Nutt.)  Kuntze  on  Pinus  contorta. 
R.  douglasii  (Engelm.)  Kuntze  on  Pseudotsuga  taxifolia. 
R.  douglasii,  var.  abietina  Engelm.,  on  Abies  grandis  and  A.  lasiocarpa. 
R.  douglasii,  var.  microcarpa  Engelm.,  on  Picea  engelmanni. 

A  glance  at  the  foregoing  list  shows  a  very  interesting  association  of 
mistletoes.  The  form  on  A.  lasiocarpa  (PI.  I/V,  fig.  3),  as  known  to  the 
writer,  in  point  of  morphology,  color,  and  the  time  of  maturity  of  pollen 
and  seed,  coincides  with  the  form  on  A.  grandis  (PL  LV,  fig.  2).  The 
mistletoe  on  Picea  engelmanni  is  slightly  smaller,  often  very  much  so 
(PL  LV,  fig.  4),  but  its  other  characteristics  are  the  same.  Comparing 
these  mistletoes  with  R.  douglasii  and  R.  pusilla,  there  is  at  once  a  marked 
similarity  among  all  five.  They  do  not  vary  widely  in  form  and  color 
of  the  stems.  There  is  some  variation  in  point  of  distribution  of  the 
individual  plants  on  the  branch,  whether  aggregated  or  appearing  singly. 
Any  one  mistletoe,  however,  may  exhibit  both  or  either  condition.  The 
staminate  flowers  of  all  five  are  a  deep  rich  purple.  No  other  species  of 
the  genus  possesses  this  character  to  such  a  marked  degree.  All  five 
bloom  at  the  same  time  in  the  same  latitude  and  exposure,  and  the  seeds 
ripen  and  are  expelled  in  the  same  month. 

The  question  naturally  arises,  What  is  the  true  taxonomic  position 
of  these  mistletoes?  Engelmann  recognized  the  close  affinities  of  the 
small  forms  on  spruce  and  fir  to  R.  douglasii  and  named  them  varie- 
ties of  that  species.  The  isolated  and  infrequent  occurrence  of  these 
small  mistletoes  on  spruce  and  fir  in  the  West  should  throw  some  light 
on  their  probable  relationships.  If  they  are  specifically  distinct,  they 
should  show  greater  activity  in  attacking  their  hosts.  As  it  is,  a  single 
tree  will  bear  a  few  plants  (broom  formation)  and  the  most  diligent  search 
on  the  same  host  for  miles  around  will  not  reveal  a  second  infection. 
The  discovery  of  W.  arceuthobii  on  those  forms  or  species  of  the  same 


July  is,  1915  W allrothiella  Arceuthobii  373 

genus  in  the  West  which  are  most  similar  to  the  eastern-spruce  mistle- 
toe may  have  some  bearing  on  the  taxonomic  position  of  this  group 
of  mistletoes.  The  occurrence  of  the  fungus  on  R.  americana,  a  very 
definitely  associated  and  characteristic  species  with  no  affinity  whatever 
with  the  mistletoes  of  the  Pseudotsuga-Abies-Picea  group,  indicates  a 
cosmopolitan  character  for  the  disease.  To  determine  this  point,  the 
fungus  has  been  introduced  into  clumps  of  the  yellow-pine  and  larch 
mistletoes.  These  experiments  are  now  under  way. 

MORPHOLOGY 

Photographs  of  W.  arceuthobii  have  not  been  published.  For  this 
reason  detail  enlargements  from  the  original  negatives  of  infected  and 
uninfected  fruits  of  R.  americana  are  reproduced  in  Plate  LVI,  figures 
i  and  2.  Reproductions  of  photographs  of  the  fungus  (natural 
size)  on  all  its  western  hosts,  so  far  as  known,  are  likewise  shown 
(PI.  L-V,  fig.  1-5).  These  illustrations  indicate  very  clearly  the  inter- 
esting habitat  of  the  fungus.  A  study  of  the  enlargements  (PI.  LVI, 
fig.  2)  shows  the  shiny  black  perithecia  densely  crowded  at  the  apex  of 
the  fruit.  Varying  numbers,  sometimes  as  many  as  40  or  more,  have 
been  counted  springing  from  the  brownish  black  stroma  within  the  seed 
capsule.  The  general  shape  of  the  perithecia  is  that  of  an  oblong  cylinder. 
Usually,  however,  they  are  slightly  enlarged  at  the  free  ends  and  very 
abruptly  rounded.  The  hyphae  composing  that  part  of  the  stroma  from 
which  the  perithecia  take  their  origin  are  densely  compacted,  brown  or 
black,  with  thick  walls.  Deeper  within  the  capsule,  the  brown  color  is 
not  so  conspicuous,  although  the  mycelium  is  generally  brownish.  The 
outer  walls  of  the  perithecia  are  uniformly  smooth;  very  rarely  a  4-  to 
6-celled  projection  is  present.  The  crowded  condition  of  the  perithecia 
often  gives  them  the  appearance  of  being  partially  embedded  in  the 
stroma.  The  wall  between  two  perithecia  when  densely  crowded  may 
be  very  thin  and  appears  to  be  occasionally  ruptured  on  the  escape  of 
the  spores  from  the  asci.  The  asci  show  considerable  variation  in  shape, 
owing  principally  to  their  crowded  condition,  but  when  free  are  uni- 
formly pear-  or  club-shaped,  with  fairly  long  pedicels.  Probably  in  no 
other  species  of  the  order  is  the  early  disappearance  of  the  wall  of  the 
ascus  so  characteristic.  Before  the  spores  have  reached  maturity  or  at 
least  before  they  have  assumed  the  normal  color  of  mature  spores,  the 
ascus  wall  disappears.  The  ascus  is  probably  ruptured  at  the  apex  by 
the  pressure  of  the  developing  spores  within.  That  a  considerable  pres- 
sure must  be  exerted  against  the  walls  of  the  ascus  is  shown  by  the  fact 
that  the  spores  when  free  are  normally  spherical,  but  within  the  ascus 
they  are  often  bluntly  angular.  They  often  persist  in  clumps  after  their 
escape  from  the  ascus.  The  asci  vary  but  very  little  in  size.  The 
measurements  (Zeiss  filar  micrometer  with  No.  12  compensating  ocular 


374  Journal  of  Agricultural  Research  vol. iv, NO. 4 

and  8  mm.  n.  a.  0.65  apochromatic  objective)  show  a  close  uniformity 
to  those  of  the  type  material.  The  measurements  of  the  asci  from  fresh 
material  range  as  follows:  22.3,  22.8,  24,  24.4,  24.8,  25.2/1  in  length. 
Evidently  considerable  shrinkage  takes  place  in  stained  material,  the 
stained  asci  measuring  16.5,  16.9,  19.8,  21.9^  in  length.  The  average 
breadth  of  the  ascus  is  3/t.  The  ascus  contains  eight  unicellular,  globose, 
thick -walled  spores.  The  spores  are  at  first  hyaline,  but  nearing  maturity 
they  assume  a  very  conspicuous  brown-black  color.  The  color  of  the 
mature  spores  is  assumed  after  their  escape  from  the  ascus.  The  pre- 
liminary color  changes  may,  however,  take  place  within  the  ascus. 
Prof.  Peck,  in  his  original  description,  states  that  the  spores  are  hyaline; 
still  he  represents,  in  his  illustration  (PI.  LV,  fig.  14),  four  mature  spores 
which  are  black.  The  change  from  a  hyaline  to  the  pronounced  brown 
or  black  color  was  evidently  recognized,  since  "an  ascus  containing 
young  spores"  is  represented,  after  which  "four  mature  spores"  that 
are  black  are  represented.  An  examination  of  some  of  the  type  material 
kindly  sent  the  writer  by  Mr.  H.  D.  House  shows  the  spores  in  all  stages 
of  development  and  varying  from  hyaline  to  black.  The  dimensions  of 
the  spores  in  the  type  material  are  found  to  agree  with  the  measurements 
of  the  spores  in  the  western  fungus,  which  range  as  follows:  Unstained 
and  out  of  ascus,  3.7,  4.5,  4.9,  5.3,  5.8,  6.2/z;  stained,  4.1,  4.5,  4.9,  5.3, 
6.2/1.  Previous  accounts  give  the  diameter  of  the  spores  as  "  about  4/t." 
The  paraphyses  are  filamentous,  short,  and  very  inconspicuous. 

All  the  asci  of  a  single  perithecium  do  not  mature  their  spores  together; 
instead,  at  the  time  mature  spores  are  escaping  from  the  perithecium, 
young  asci  showing  early  stages  of  spore  differentiation  are  discernible. 
There  is  consequently  a  gradual  dissemination  of  the  spores,  governed 
to  an  extent  by  the  humidity  of  the  atmosphere.  The  opening  through 
which  the  spores  escape  is  directly  at  the  apex  and  is  formed  by  the  free 
ends  of  the  thick-walled  hypha  composing  the  walls  of  the  perithecium. 
The  cells  composing  the  tips  of  these  hyphae  seem  to  possess  certain 
hygroscopic  properties,  as  they  are  observed  to  bend  in  or  out  on  the 
addition  or  absence  of  moisture. 

BIOLOGY 

In  what  manner  the  spores  of  W.  arceuthobii  are  conveyed  to  the 
pistillate  flower  of  the  mistletoe  in  nature  the  writer  is  not  in  a  posi- 
tion to  state  definitely.  Since  isolated  infections  occur  promiscously 
on  different  branches  of  the  same  tree  or  on  different  trees  in  the  same 
locality,  it  is  evident  that  the  wind  is  the  chief  factor  in  spore  dissemina- 
tion. A  fact  observed  among  the  infected  plants  on  lodgepole  pine  of 
the  swamp  area  previously  mentioned  supports  this  view.  The  area 
lay  with  its  long  axis  in  the  direction  of  the  prevailing  winds  of  the  Priest 
River  Valley.  An  examination  of  the  trees  bearing  infected  plants 
showed  that  they  were  more  or  less  in  line  with  each  other  and  extended 


juiy  i5, 1915  Wallrothiella  Arceuthobii  375 

in  the  direction  of  the  most  constant  winds.  On  either  side  of  the  most 
heavily  infected  area  the  trees  did  not  support  infected  plants,  although 
the  mistletoe  was  abundant.  Furthermore,  large  compact  brooms 
always  bore  the  greater  number  of  infected  plants  on  the  windward  side. 

The  writer  has  recently  determined  that  insects  to  a  certainty  play 
a  role  in  the  pollination  of  these  mistletoes.  Hymenopterous  insects 
are  chiefly  in  evidence,  but  those  of  other  orders  are  also  known  to 
promote  pollination.  During  1914  grasshoppers  in  great  numbers 
came  out  of  the  Hangman  Creek  Valley  near  Spokane  and  fed  upon 
blooming  staminate  plants  of  the  large  mistletoe  growing  in  profusion 
on  yellow  pine  of  the  bench  lands.  These  insects  seemed  to  select  only 
the  flowers  of  the  staminate  plants  for  food;  but,  swarming  over  the 
pistillate  plants,  they  deposited  some  of  the  pollen  that  adhered  to  their 
bodies.  It  is  as  easily  possible  that  the  spores  of  the  mistletoe  fungus 
are  in  a  minor  degree  transported  in  a  like  manner.  Rain  dropping  from 
infected  to  uninfected  plants  or  running  down  the  pendent  branches 
and  dropping  off  at  the  tips  of  the  mistletoe  plants  is  probably  a  factor 
in  distributing  the  disease  on  any  one  tree  or  broom.  It  so  happened 
at  the  field  station  that  a  number  of  newly  collected  infected  capsules 
were  left  overnight  and  a  portion  of  the  following  day  on  a  glass  slide 
under  the  microscope.  An  examination  of  the  slide  showed  that  a  num- 
ber of  spores  had  been  expelled  and  lay  in  a  ring  about  ~%  mm.  away  from 
the  apex  of  the  perithecium.  Evidently  there  is  a  slight  expulsion  of  the 
spores  under  favorable  conditions.  This  came  as  a  surprise,  as  the  stiff 
ends  of  the  hyphae  forming  the  perithecial  wall  seem  to  open  with  diffi- 
culty. A  number  of  perithecia  collected  from  fallen  capsules  in  the 
spring  still  contained  numerous  spores.  The  early  disappearance  of  the 
ascus  within  the  perithecium  precludes  any  expulsion  from  this  source. 
The  force  must  arise  from  the  continual  maturing  and  crowding  of  the 
spores  toward  the  outward  end  of  the  perithecium.  Under  favorable 
conditions  this  pressure  may  become  sufficient  to  force  the  spores  out 
through  the  aperture.  It  has  already  been  indicated  that  a  pressure 
seems  to  exist  within  the  perithecium.  This  force,  though  weak,  may 
still  be  sufficient  to  cause  the  spores  to  land  on  capsules  of  the  same  plant 
that  escaped  previous  infection. 

The  spores  of  the  fungus  are  beginning  to  ripen  and  to  be  expelled 
from  the  perithecia  in  the  latitude  of  northern  Idaho  about  the  end  of 
November  and  are  capable  of  germinating  immediately.  The  method 
of  penetration  of  the  germ  tube  of  the  spore  into  the  developing  fruit  of 
its  host  has  not  as  yet  been  observed.  Since  a  considerable  period 
elapses  between  pollination  and  the  time  actual  fertilization  takes  place 
in  the  host,  it  is  quite  possible  that  the  germination  of  the  fungus  spore 
coincides  with  the  advance  of  the  pollen  tube  toward  the  embryo  sac. 
This  would  enable  the  germ  tube  of  the  spore  to  travel  toward  the  ovule 
of  its  host  by  a  line  of  least  resistance.  In  early  spring,  or  at  the  time 


Journal  of  Agricultural  Research  vol.  iv,  NO.  4 

actual  fertilization  of  the  mistletoe  takes  place,  those  tissues  destined  to 
become  the  seed  are,  in  infected  plants,  observed  to  be  completely  filled 
or  destroyed  by  the  mycelium  of  the  fungus.  After  infection,  the  young 
seed  capsule  never  increases  much  in  size  and  is  entirely  dominated  by 
the  parasite.  The  diseased  capsules  usually  fall  away  during  late  winter 
and  early  spring,  which  allows  time  for  the  infection  of  the  pistillate 
plants.  The  drain  on  the  vigor  of  the  mistletoe  plant,  if  all  the  young 
capsules  are  infected,  is  such  that  it  may  also  succumb  and  fall.  If  only 
one  or  two  capsules  of  the  plant  are  infected,  it  will  remain  intact, 
maturing  the  uninfected  fruit  of  the  season  and  fruiting  again  the  follow- 
ing year.  Usually,  however,  the  infection  of  all  the  fruits  of  a  mistletoe 
colony  or  of  all  the  plants  of  a  broom  is  so  complete  that  few  or  no  seeds 
mature. 

ECOLOGY 

All  collections  so  far  made  of  the  fungus  have  not  been  at  an  elevation 
much  greater  than  3,600  feet,  although  its  hosts  may  range  well  up  toward 
the  timber  line.  This  indicates  a  preference  for"  the  conditions  of  the 
lower  levels,  where  it  is  not  so  much  exposed  to  fluctuations  of  warmth 
and  moisture.  The  latter  factor  is  probably  of  greater  influence.  Until 
the  fungus  is  found  elsewhere  it  may  be  said  to  prefer  the  North  Tem- 
perate regions.  Forestburg,  N.  Y.,  its  first  known  station,  is  about  on 
a  line  with  the  Upper  Peninsula  of  Michigan,  the  region  of  its  second  dis- 
covery, and  northern  Idaho,  where  it  was  last  found.  This  is  its  geo- 
graphical and  climatic  range  at  present.  Developing  either  on  exposed 
or  shaded  plants,  the  fungus  seems  to  favor  those  growing  in  shaded 
positions,  such  as  the  inner  parts  of  brooms.  Absence  of  direct  sun- 
light may  promote  development,  but,  after  the  capsule  becomes  infected, 
direct  sunlight  can  not  have  much  influence  on  the  maturing  of  the 
fungus.  The  germination  of  the  spores  would  probably  be  promoted  by 
an  absence  of  direct  sunlight.  Warm  fall  rains,  such  as  occur  in  northern 
Idaho,  are  undoubtedly  very  favorable  to  the  development  and  spread 
of  the  disease,  since  in  this  region  the  fungus  has  been  found  most  abun- 
dant. In  damp  river  bottoms  or  on  the  borders  of  swamp  areas  the  lodge- 
pole-pine  mistletoe,  which  frequently  occurs  in  profusion  in  such  a  habi- 
tat, is  very  likely  to  be  attacked  by  the  fungus.  Prof.  Peck  *  does  not 
record  the  conditions  under  which  the  fungus  was  growing  at  Forest- 
burg,  N.  Y.,  but  presumably  it  was  a  region  of  considerable  humidity. 
The  Upper  Peninsula  of  Michigan,  where  Prof.  Wheeler  collected  the 
fungus,  is  a  region  of  numerous  swamps  and  abundant  atmospheric 
moisture.  In  view  of  the  fact  that  the  fungus  is  parasitic  on  the  rather 
succulent  capsule  of  the  mistletoe,  atmospheric  humidity  should  not 
greatly  interfere  with  its  life  functions,  except  probably  in  the  initial 
stages  of  spore  germination.  The  fungus  should  thrive  on  the  larch 

1  Peck,  C.  H.     Op.  cit. 


July  is,  1915  W allrothiella  Arceuthobii  377 

mistletoe,  provided  it  is  susceptible  to  attack,  owing  to  the  usually  damp 
condition  of  the  compact  moss-covered  brooms.  It  remains  to  be  seen 
under  just  what  conditions  the  fungus  will  propagate  itself.  To  this  end 
it  is  being  introduced  into  mistletoe  regions  of  all  types  of  exposure. 

The  ease  with  which  the  fungus  seems  to  infect  its  host  leads  the  writer 
to  believe  that  it  may  be  of  some  economic  importance  in  the  control  of 
certain  species  of  mistletoe,  at  least  for  small  areas.  For  a  mistletoe 
species  to  propagate  itself,  it  must  produce  seeds  abundantly,  in  order 
to  insure  the  infection  of  the  young  growing  forest.  The  proportion  of 
mistletoe  seeds  actually  causing  infection  to  the  total  number  produced 
is  very  small  indeed.  Some  fall  to  the  ground;  some  fall  on  plants  not 
susceptible;  most  of  them  fall  on  parts  of  the  host  too  old  to  be  pene- 
trated by  the  young  root  of  the  seed.  With  the  exception  of  a  few  rare 
instances,  where  infections  have  been  known  to  occur  on  wound  tissue 
of  mature  parts  of  trees,  the  writer  has  not  yet  found  either  in  nature  or 
by  actual  inoculation  a  seed  taking  effect  on  any  part  of  its  host  other 
than  the  more  tender  shoots  or  their  equivalents  in  tenderness  of  bark 
and  then  only  when  the  primary  sinker  found  its  way  to  a  leaf  scar,  leaf 
scale,  or  other  more  vulnerable  irregularities  of  the  substratum.  Again, 
the  seed  must  fall  in  such  a  position  that  the  protruding  root  may  directly 
find  its  way  under  a  leaf  scale  or  be  sheltered  by  the  thick  bunch  of 
needles  at  each  node  of  growth  or  at  the  base  of  a  leaf  or  leaf  sheath; 
otherwise  it  may  fail  of  its  purpose.  The  seed  may  germinate  and  expend 
its  stored  materials  in  the  production  of  a  primary  root  of  half  an  inch 
or  more,  but  before  the  growing  point  can  penetrate  the  stem,  provided 
it  is  in  such  a  position  as  to  be  drawn  toward  it,  the  young  hypocotyl  is 
exhausted.  Very  few  seeds  cause  an  infection  when  not  very  favorably 
located  or  directly  through  the  smooth  epidermis  possessing  a  suberized 
layer. 

With  the  exception  of  the  small  forms  mentioned  in  this  paper  most  of 
the  members  of  the  genus  are  prolific  seed  producers.  If  so  few  seeds  find 
a  vulnerable  point  on  their  hosts  even  with  an  abundant  production  of 
seed,  so  much  less  will  the  chances  of  infection  be  if  the  seed  production 
is  lessened.  An  estimate  of  the  number  of  seed  that  should  have  been 
produced  by  the  lodgepole-pine  mistletoe  on  a  small  broom  was  about 
400.  Not  a  single  mistletoe  seed  on  this  broom  had  reached  maturity. 
All  were  attacked  by  the  fungus.  The  biologic  control  of  organic 
agents  destructive  to  plant  life  is  in  most  cases  a  thing  very  much  in 
the  realm  of  fancy.  It  seems,  however,  that  a  fungus  of  the  nature 
of  W.  arceuthobii  may  be  introduced  into  mistletoe  regions  possessing 
certain  climatic  conditions  with  the  prospect  of  reducing  the  seed  pro- 
duction of  these  parasites,  and  thus  reducing  the  damage  caused  by 
the  mistletoe. 


Journal  of  Agricultural  Research  voi.iv,No.4 

SUMMARY 

Wallrothiella  arceuthobii,  a  fungous  parasite  on  the  false  mistletoes  of 
conifers,  is  reported  for  the  first  time  in  the  West. 

This  fungus,  first  collected  by  Prof.  Peck  in  New  York  and  again  by 
Prof.  Wheeler  in  the  Upper  Peninsula  of  Michigan,  was  considered  a  very 
rare  species  until  it  was  found  to  be  of  common  occurrence  in  parts  of 
Montana  and  Idaho. 

Several  new  facts  pertaining  to  the  morphology  and  general  behavior 
of  the  fungus  are  established. 

Its  host  range  has  been  greatly  extended. 

The  significant  fact  that  the  fungus  is  found  in  the  West  on  those  forms 
of  species  of  the  same  genus  which  are  most  similar  to  the  eastern  black- 
spruce  mistletoe,  its  host  in  the  East,  is  thought  to  have  some  bearing 
on  the  taxonomic  position  of  this  particular  group  of  mistletoes. 

Its  parasitism  on  the  false  mistletoes  is  found  to  be  of  great  significance 
in  the  control  of  these  parasites,  which  are  so  destructive  to  many  western 
conifers. 


PLATE  LV 

Fig.  i. — Razoumofskya  douglasii  on.  Pseudotsuga  taxifolia,  infected  with  Wallroihiella 
arceuthobii.  Note  that  two  capsules  escaped  infection.  Natural  size. 

Fig.  2. — R.  douglasii,  var.  abietina,  on  Abies  grandis,  infected  with  W.  arceuthobii. 
Natural  size. 

Fig.  3. — R.  douglasii,  var.  abietina,  on  Abies  lasiocarpa,  infected  with  W.  arceuthobii. 
Natural  size. 

Fig.  4. — R.  douglasii,  var.  microcarpa,  on  Picea  engelmanni,  infected  with  W. 
arceuthobii.  Natural  size. 

Fig.  5. — Left  and  right  figures  showing  infection  of  R.  americana  with  W.  arceuthobii 
by  infected  plants  of  R.  douglasii.  The  plants  at  lower  part  of  figures  are  normal  and 
fully  mature.  The  middle  figure  shows  infection  of  R.  americana  by  spraying  upon 
the  plants  a  mixture  containing  spores  of  W.  arceuthobii.  Natural  size. 


Wallrothiella  Arceuthobii 


PLATE  LV 


i 


Journal  of  Agricultural  Research 


Vol.  IV,  No.  4 


Wallrothiella  Arceuthobii 


PLATE  LVI 


Journal  of  Agricultural  Research 


Vol.  IV,  No.  4 


PLATE  LVI 

Fig.  i. — Enlargement  of  the  normal  fruits  of  Razoumofskya  americana  shown  in 
Plate  LV,  figure  5. 

Fig.  2. — Enlargement  of  the  diseased  fruits  of  R.  americana  infected  with 
Wallrothiella  arceuthobii  shown  in  Plate  LV,  figure  5.  Both  plants  are  enlarged  to 
the  same  scale  and  show  the  proportionate  size  of  infected  and  normal  mature  fruits. 
92315°— 15 8 


S^LLIBRARIES 


